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Wrap image around cylinder using canvas drawImage opacity issue

I need to wrap an image around another image of a mug using javascript, and I found this:
Wrap an image around a cylindrical object in HTML5 / JavaScript
This helps when loading the image that has the mug handle on the left. However when using the same function (with tweaked position values) the image has an opacity applied to it. I searched endlessly to figure out for what reason this is happening however I found nothing :/
This is the function used to wrap the image for the mug with the right handle:
function canvas2() {
var canvas = document.getElementById('canvas2');
var ctx = canvas.getContext('2d');
var productImg = new Image();
productImg.onload = function() {
var iw = productImg.width;
var ih = productImg.height;
canvas.width = iw;
canvas.height = ih;
ctx.drawImage(
productImg,
0,
0,
productImg.width,
productImg.height,
0,
0,
iw,
ih
);
loadUpperIMage();
};
productImg.src =
'https://i.ibb.co/B2G8y1m/white-right-ear.jpg';
function loadUpperIMage() {
var img = new Image();
img.src =
'https://i.ibb.co/BnQP0TL/my-mug-image.png';
img.onload = function() {
var iw = img.width;
var ih = img.height;
var xOffset = 48, //left padding
yOffset = 68; //top padding
var a = 70; //image width
var b = 8; //round ness
var scaleFactor = iw / (6 * a);
// draw vertical slices
for (var X = 0; X < iw; X += 1) {
var y = (b / a) * Math.sqrt(a * a - (X - a) * (X - a)); // ellipsis equation
if (!isNaN(y)) {
ctx.drawImage(
img,
X * scaleFactor,
0,
iw / 0.78,
ih,
X + xOffset,
y + yOffset,
1,
162
);
}
}
};
}
}
Hope someone can help with this!
Here is a fiddle with the issue https://jsfiddle.net/L20aj5xr/

It is because of the 4th argument you pass to drawImage - iw / 0.78. By multiplying image width by a value lower than one, you get the value larger than image width. The spec for drawImage says:
When the source rectangle is outside the source image, the source rectangle must be clipped to the source image and the destination rectangle must be clipped in the same proportion.
ctx.drawImage(image, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight);
Because the source width (sw) you are using is larger than source image size, the destination rectangle "is clipped in the same proportion". The destination rectangle width is 1px because you chose it as a width for each vertical line you are drawing, and after clipping it's width becomes 1 * 0.78 = 0.78px. The width is now less than 1px and to be honest I am not exactly sure how it actually works under the hood, but my guess is that a browser still needs to draw that 1px, but because the source is 0.78px, it kinda stretches the source to that 1px and adds some anti-aliasing to smooth the transition, which results into added transparency (i.e. browser does not have enough information for that 1px and it tries to fill it up the best it can). You can play around with that by incresing sw even more and observe increasing transparency.
To fix your issue I used the value 20 instead of 0.78 like for the first cup and it seemed to look ok.

Javascript - set context2d Image as background for canvas

I'm using the below code to set the good quality image as background for canvas. If I set the image directly as background, it was working. But the image quality isn't good. So I'm using another canvas to have a good quality image and then I need to set as background. I'm using fabricjs
var _img = new Image();
_img.src = img_base64;
var ctx = canvas.getContext("2d");
_img.onload = function(){
// set size proportional to image
canvas.height = canvas.width * (_img.height / _img.width);
// resize to 50%
var oc = document.createElement('canvas'),
octx = oc.getContext('2d');
oc.width = _img.width * 0.5;
oc.height = _img.height * 0.5;
octx.drawImage(_img, 0, 0, oc.width, oc.height);
octx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5);
ctx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5, 0, 0, canvas.width, canvas.height);
// Should be an instance of fabric.image
var imgInstance = new fabric.Image(ctx, {
width: canvas.width,
height: canvas.height
});
console.log(imgInstance);
canvas.setBackgroundImage(imgInstance, canvas.renderAll.bind(canvas), {
width: canvas.width ,
height: canvas.height,
/*originX: 'left',
originY: 'top'*/
// Needed to position backgroundImage at 0/0
});
};
The error is
Argument 1 of CanvasRenderingContext2D.drawImage could not be converted to any of: HTMLImageElement, SVGImageElement, HTMLCanvasElement, HTMLVideoElement, ImageBitmap
I'm using base64 actually. but for the fiddle, I'm using direct path
Here is the Fiddle
Edit : I see the image rendering in my local, But I need to set it as background.

The only reason why an image quality may not be good is because there is an high downscale/upscale factor.
Downscaling is good with image and css elements, while is bad when done on drawImage on the canvas, since the canvas use a nearest neighbour resize.
Other than that, quality of the image is preserved.
Fabricjs try to help on this offering resize filters.
You can apply a resize filter to an image when you load it so that then you work with an image of the dimension you need and that match 1:1 your canvas pixels.
Be aware that then if you export in higher resolution you will not get back the original quality of the image.

How can i resize my canvas image

page 1:
//Calls the function from page 2 and the callback image is set as the source of the control.
previewImage(current, function(img) {
jQuery(".mapItem").attr("src",img.src);
});
page 2:
//The functions callback returns an image which we use in page 1 (above)
var canvas = document.createElement('canvas');
context = canvas.getContext('2d');
context.drawImage(this.m_Images[i],0,0,canvas.width,canvas.height);
var t = new Image();
t.src = canvas.toDataURL();
callback(t);
The issue:
I have 2 JavaScript pages, the first one has an image control and the second one has a function that returns a callback as an image.
My control in page 1 (.mapItem) has a height and width of 75.2px (fixed). The image that is coming from the callback however will have a different size each time e.g one day it can be 200px * 300px and one day it can be 150px * 200px etc
How can I clip or CUT the image of the callback? I want the image (t) to zero (0) as x and y starting points and then clip the image where ever the .mapItem control height and width is.
I need this to be proportional ratio. So I can't just add the following code:
context.drawImage(this.m_Images[i],0,0,72.5,72.5); because this will ruin the image as we dont even know if it is square shaped.
Thanks in advance :)

You can determine the proportions of callback image and then apply them to the page 1 image thus:
Let's assume that the callback image is 300x200px. The ratio of the image's height-to-width can be expressed as...
var ratio = callbackImage.height / callbackImage.width;
...or, in this case...
var ratio = 200 / 300; // 0.666...
We know the width of the page 1 canvas is 72.5 so we can apply the ratio to that value to determine the proportional height of the callback Image like so...
var canvasWidthHeight = 72.5;
var imageHeight = canvasWidthHeight * ratio; // 48.333...
To center the callback Image on the page 1 canvas calculate the y offset like so..
var y = (canvasWidthHeight - imageHeight) / 2;
...and now you can use the canvas drawImage method with these values...
context.drawImage(
this.m_Images[i],
0, y,
canvasWidthHeight, imageHeight
);
If the callback image was higher than it was wide you'd apply the ratio the page 1 canvas dimensions to work out the x offset rather than the y offest. If the callback image was square then its ratio would be 1.0 and you could simply paint it into the square page 1 canvas at
context.drawImage(
this.m_Images[i],
0, 0,
canvasWidthHeight, canvasWidthHeight
);
All together the code might look something like this...
var image = this.m_Images[i];
var canvas = {
width: 72.5,
height: 72.5
};
var wh = 0;
var ratio = image.height / image.width;
if (image.width > image.height) { // landscape
wh = canvas.width * ratio;
context.drawImage(
image,
0, (canvas.height - wh) / 2,
canvas.width, wh
);
} else if (image.width < image.height) { // portrait
ratio = image.width / image.height;
wh = canvas.height * ratio;
context.drawImage(
image,
(canvas.width - wh) / 2, 0,
wh, canvas.height
);
} else { // square
context.drawImage(
image,
0, 0,
canvas.width, canvas.height
);
}
Hope that helps. ;)
EDIT: You may need to ensure that the new Image() has fully loaded before initiating the callback. You can do this with the following snippet...
// callback preparation code as before...
var t = new Image();
t.addEventListener('load', function() {
callback(this);
});
t.src = canvas.toDataURL();

Canvas ImageData don't get more than 102,000 values?

I am getting pixels values from an image using canvas. The image size is 170*170 pixels. Here is my code:
var canvas = document.createElement("canvas");
canvas.style.width = img.width;
canvas.style.height = img.height;
canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);
var pixelData = canvas.getContext('2d').getImageData(0, 0, img.width, img.height).data;
It works well, I have values in pixelData, until I reach pixelData[102000]... I've test it with a white image, and all the values from pixelData[0] to pixelData[101999] are 255, but then it is 0 until the end...
Somebody sees why? Maybe this is about canvas width and height?

Your canvas size is not what you think it is.
You are only setting the size of the canvas element not the canvas bitmap:
canvas.style.width = img.width;
canvas.style.height = img.height;
This means your bitmap is actually 300 x 150 pixels in size, the default size, and you're just scaling that to the size of the image (since it's all white you won't be able to detect this so easily).
Since your image is 170 x 170 pixels you will only paint part of the canvas leaving the rest to default RGBA value [0,0,0,0].
In order to properly set the size of the canvas you must edit the above mentioned lines to be:
canvas.width = img.width;
canvas.height = img.height;

HTML Canvas: How to draw a flipped/mirrored image?

I'm trying to flip/mirror an image as I paint it on an HTML canvas; I found a game tutorial showing a sprite sheet per direction a character has to face, but this doesn't seem quite right to me. Especially since each frame has a different size.
What would be the best technique to reach this goal?
I tried to call the setScale(-1, 1); on my canvas with no success. Maybe that isn't meant for this.
Thanks

You can do this by transforming the context with myContext.scale(-1,1) before drawing your image, however
This is going to slow down your game. It's a better idea to have a separate, reversed sprite.

You need to set the scale of the canvas as well as inverting the width.
drawToCanvas : function(v, context, width, height){
context.save();
context.scale(-1, 1);
context.drawImage(v, 0, 0, width*-1, height);
context.restore();
}
There are probably some performance issues with this but for me that was not an issue.

If you just flip it horizontally it will get off of bounds... so use translate to adjust its position:
ctx.translate(canvas.width, 0);
ctx.scale(-1, 1);
ctx.drawImage(img, 0, 0);
For a shorter code you can remove the translate and use the image size as negative offset in the second parameter of the drawImage (x coordinate) instead:
ctx.scale(-1, 1);
ctx.drawImage(img, canvas.width * -1, 0);
If you want to restore the context later, add save/restore before and after it all:
ctx.save();
ctx.scale(-1, 1);
ctx.drawImage(img, canvas.width * -1, 0);
ctx.restore();

You don't need to redraw the entire image when creating a reflection. An original reflection simply shows the bottom part of the image. This way you are redrawing a smaller part of the image which provides better performance and also you don't need to create linear gradient to hide the lower part of the image (since you never draw it).
var img = new Image();
img.src = "//vignette2.wikia.nocookie.net/tomandjerryfan/images/9/99/Jerry_Mouse.jpg/revision/latest?cb=20110522075610";
img.onload = function() {
var thumbWidth = 250;
var REFLECTION_HEIGHT = 50;
var c = document.getElementById("output");
var ctx = c.getContext("2d");
var x = 1;
var y = 1;
//draw the original image
ctx.drawImage(img, x, y, thumbWidth, thumbWidth);
ctx.save();
//translate to a point from where we want to redraw the new image
ctx.translate(0, y + thumbWidth + REFLECTION_HEIGHT + 10);
ctx.scale(1, -1);
ctx.globalAlpha = 0.25;
//redraw only bottom part of the image
//g.drawImage(image, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight);
ctx.drawImage(img, 0, img.height - REFLECTION_HEIGHT, img.width, REFLECTION_HEIGHT, x, y, thumbWidth, REFLECTION_HEIGHT);
// Revert transform and scale
ctx.restore();
};
body {
background-color: #FFF;
text-align: center;
padding-top: 10px;
}
<canvas id="output" width="500" height="500"></canvas>